Create a Firebase project here: https://firebase.google.com/. You'll need a
signed Android build for that, that can be a debug self-signed build too, just
retrieve the signing hash. The key hash of an already signed ap can be obtained
as follows (on macOS): keytool -list -printcert -jarfile the-app.apk
Place the generated google-services.json file in android/app
for Android and the GoogleService-Info.plist into ios/app for
iOS (you can stop at that step, no need for the driver and the code changes they
suggest in the wizard).
You may want to exclude these files in YOUR GIT config (do not exclude them in
the .gitignore of the application itself!).